This invention relates, in general, to scanning mass spectrometers which have an electro-optical ion detector located at an angle to the exit plane of the magnetic sector of the mass spectrometer and which comprises a channel electron multiplier assembly to which a twisted fiberoptic window is optically coupled for transferring light energy generated by the ions to a photo diode detector to enable detection of ions over a wide mass range. Such as electro-optical ion detector in a scanning mass spectrometer is disclosed in the parent application and this invention is particularly directed to an improvement in the optical coupling between the channel electron multiplier assembly and the twisted fiberoptic window. This invention also includes the method of making such an improved electro-optical ion detector.
The parent application also disclosed a method of making the twisted fiberoptic window in such a manner that the entrance end of the twisted fiberoptic window could be placed parallel to the image intensifier plate and parallel to the photodiode array which, in turn, was parallel to the exit plane of the magnetic sector of the mass spectrometer. The reason for the configuration of the twisted fiberoptic window is fully explained in the parent application.
The electro-optical ion detector of the parent application performs very well but it was found that the spacing between the channel electron multiplier assembly and the entrance of the twisted fiberoptic window, while known to be important, was difficult to achieve with any consistency. If the spacing was not precise, Moire patterns were formed which obscured the data being measured. To improve this spacing between the channel electron multiplier assembly and the entrance to the twisted fiberoptic window so that the spacing is consistent and easily obtained is the main thrust of this invention. This invention also teaches a method of making such a space easily obtainable.
While the following background information was fully disclosed in the parent application, for convenient reference, most of the disclosure in the parent application is reproduced hereinafter and where possible the same reference numerals are used in this application as used in the parent application for the same reason. However, reference should be made to the parent application for any necessary clarification.
Attention is directed to FIGS. 1-4 which are disclosed in the parent application.
FIG. 1 shows an artist rendition of a prior art double focus scanning mass spectrometer 10 comprising a sample inlet 12 having an ion source 14 with an ionizing region 16 in which sample gas molecules are subjected to an electron beam 20 to form an ion beam 22. The ion beam 22 is directed through an electrostatic section 24 and a magnetic sector 26. Ions traveling through the magnetic sector are scanned as at 30 and those of a selected mass are focused at focal point 32 and directed through a narrow slit 34 to an electron multiplier 36.
In this instrument, ions of slightly higher and lower mass focus along a nearly straight line focus, i.e., focal plane 44, which passes through the axial focal point 32 and which lies at an angle to the axis 34. This focal plane 44, called an angled focal plane, is illustrated in phantom in FIGS. 1a and 1b.
As can be seen in FIG. 1b, because of the geometry of the instrument, the ions which have slightly higher and lower mass, identified therein as 32a, 32b, 32 c and 32d, by way of example, are ordinarily not detectable since they are blocked from entering the electron multiplier 36 and will continue to be blocked and undetected until the magnitude of the magnetic field is increased or decreased (also called stepped or adjusted) to bring these ions into focus and directed to the electron multiplier 36.
The invention of the parent application provided a means for detecting ions along this angled focal plane using an electro-optical ion detector with the image intensifier plate 80 of a channel electron multiplier assembly placed at the angled focal plane giving essentially simultaneous analysis of a limited range of ion masses, but switchable over several ranges for detection over a wide mass range.
As shown in FIG. 2, the microchannel or image plate 80 is a circular glass structure with a plurality of channels 82 in which straight channel electron multipliers 84 are placed and optically coupled to a fiberoptic window 86 the ends of which are coated with a phosphor to form a screen 90 as shown in FIG. 3. The components shown in FIG. 3 are suitably clamped together by bolts 92, to form a channel electron multiplier assembly 94 which operates the same as the channel electron multiplier assembly of FIG. 2. Assembly 94 is commercially available and are commonly supplied with the fiberoptic window 86 covered with the phosphor coating 90 which converts the secondary electron energy into a visible light image, which is guided through the fiberoptic window 86.
As disclosed in FIG. 4, which corresponds to FIG. 4 of the parent application, an electro-optical ion detector 100 is connected at the exit plane 102 of the magnetic sector 26 of the scanning mass spectrometer 10. The exit plane is shown schematically and also shown coupled to the top of a vacuum envelope 104 with an exit plane 106 located at the magnetic center of curvature 110 so that ions will be directed toward the surface of the image plate 80 of the channel electron multiplier assembly 94 within the vacuum element 104. The image plate 80 of the channel electron multiplier assembly 94 corresponds to the device shown in FIG. 3 and the same reference numerals are used in this figure for simplicity purposes. High and low mass ions 32a and 32c, outwardly of the central ion beam 32b, are focused and shown impinging on the surface of the image plate 80 as described in FIGS. 1a-b.
The image plate 80 is located at an angle to the exit plane 106 which corresponds to the angled focal plane 44 and is supported by a support plate 112 which, in turn, is supported by a pedestal 114 positioned on the lower body member as shown in FIG. 9. The envelope 104 is formed by two body members 116 and 120 and a flexible coupling 22 and the vacuum in the vacuum envelope is maintained by a vacuum pump 124. The two body members 116 and 120 are movable relative to one another by a plurality of adjustment screws 126 to vary the location of the image plate 80 relative to the exit plane 106. Transducers (linear variable differential transformers) 130 provide a means of precise measurement of the position of the lower body member 120 relative to the upper body member 116 from which the position of the image plate 80 may be determined.
The support plate 112 has a central rectangular shaped aperture 132 through which a twisted fiberoptic window 134 is optically coupled to the fiberoptic window 86 containing the phosphor coating 90 and extends through an opening in the vacuum flange 120. The photodiode array 140 is connected to suitable electronics 142 to provide the necessary signal generated by the detector ions. The photodiode detector 140 is disposed parallel to the exit plane 106.
As shown in this FIG. 4, the twisted fiberoptic window 114 is shaped in such a way as to enable the entrance ends of the fibers to be parallel to the angled image plate 80 and the exit ends 134 of the fibers to be positioned parallel to the exit plane 102 of the magnetic sector 26. The reason for this configuration of the twisted fiberoptic window 134 is fully explained in the parent application. Also disclosed in the parent application is the fact that the ends of the fibers of the twisted fiberoptic window 134 are ground and polished for maximum light acceptance efficiency.
Again, the optical coupling between the channel electron multiplier assembly 94 and the twisted fiberoptic window 134 is important but heretofor there was no way in which the spacing between the assembly 94 and the window 134 could be obtained with consistency for the maximum transfer of light energy from the assembly 94 to the window 134 and it is an object of this invention to provide an electro-optical ion detector with precise spacing for the maximum transfer of energy from the channel electron multiplier assembly 94 to the twisted fiberoptic window 134. Another object of this invention is a method of making an improved electro-optical ion detector for use in a mass spectrometer.